This invention relates to a priority selector for a plurality of data elements selectively seeking priority recognition.
Data communications may take many forms such as between a central processing unit (i.e. computer) and numerous input/output (I/O) devices or between a centralized telephone office and a series of telephone lines, for example.
Some prior computer systems do not provide any interrupt capability but rather continually address each device or element to determine whether it is ready to receive or transmit data.
More recent equipment permits a central processing unit to compute until notified that a device or element is ready to transmit or receive data. Some systems, however, require the computer to poll each device or element to determine which device or element is seeking the interrupt.
Other systems employ a daisy chain configuration wherein interface circuits for each device are interconnected in serial configuration so that the device connected nearest the computer would, by circuit definition, have the highest assigned priority while each subsequently connected interface circuit would have a sequentially lower priority. In such daisy chain construction, an interrupt being detected in the middle of the daisy chain would prevent recognition of a higher priority interrupt until the computer completes servicing of a previously recognized interrupt. Frequently, such systems "freeze" the operation to prevent interference from subsequently entered higher priority interrupts when the system is servicing a recognized interrupt.
Multiple daisy chains have also been utilized for priority recognition purposes. Such systems are very complex and require numerous program instructions to save, store and freeze the various interrupts as they occur which wastes computing time and computing flexibility.